Why do batteries leak?

When you open a dead
flashlight you might find a scarry multicolored mess. White powdery gunk, and
corrosion of the terminals. A carbon-zinc flashlight battery consists of a zinc
can filled with manganese dioxide and liquid. The liquid is water with either
ammonium chloride or zinc chloride dissolved in it. The battery leaks when the
zinc can is dissolved. The liquid at this point can contain ammonium chloride
and zinc chloride. There is also a starch paste used as the separator, and this
can also leak out.

When the battery is totally
discharged the reaction products include manganese hydroxide, zinc ammonium
chloride, ammonia, zinc chloride, zinc oxide, and water. So for a carbon zinc
battery the white gunk is a mixture of these compounds and starch. None of
these are toxic, in fact they can all be used in fertilizer.

But WHY do they leak? The
battery power comes from a chemical reaction which consumes the zinc. The zinc
can eventually dissolves due to the main reactions and other side reactions,
which is why it is more likely to leak when left to discharge even when it is
dead, as when your five year old leaves the flashlight on and hides it under
his bed.

Alkaline batteries are very
similar to carbon zinc batteries. They use manganese dioxide and metallic zinc
as the reactive materials, but they use an alkaline potassium hydroxide
solution for the electrolyte instead of the mildly acidic ammonium chloride.
The reaction products are manganese oxide and zinc oxide with by-products of
manganese hydroxide and zinc hydroxide. So the white residue consists of
manganese oxide, zinc oxide, potassium hydroxide, zinc hydroxide, and manganese
hydroxide. Soon after oozing out, the potassium hydroxide reacts with carbon
dioxide in the air to form potassium carbonate. Again, none of these chemicals
is toxic, and all can be used in fertilizer.

Some people will look up the MSDS and see that there is
potassium hydroxide electrolyte and warn you that this is corrosive and could
eat your skin and blind you. There isn't any free liquid, it is all absorbed
into the battery components. If you cut the battery and smeared the black
manganese dioxide powder on your hands you might get a slippery feeling as the
lye converts oils in your skin into soap. But as mentioned above, a few hours
exposure to atmospheric carbon dioxide will convert the KOH into K2CO3, which
is perfectly safe. The KOH is very soluble, but the potassium carbonate isn't
so much, which is a test to see if the reaction has occurred. In a flashlight
the ingress of CO2 is slow, so the KOH has time to corrode the innards of the
flashlight.

But WHY do alkaline batteries leak?

There are side reactions that generate hydrogen gas--basically
the zinc breaks down the water. This gas will build up in pressure and
eventually vent, driving some of the liquid electrolyte out with it.
Manufacturers at one time uses mercury to form a non-reactive skin on the zinc
to prevent this, but of course due to government mandates mercury is no longer
used in alkaline batteries.